Individuals with alcohol use problems represent a heterogeneous group with respect to variables relevant to classification and treatment. Failure to account for this heterogeneity would be expected to compromise treatment outcome and matching effects. With this in mind, Project MATCH hypothesized several treatment matching relationships based on alcoholic type, alcohol severity, psychiatric severity, and alcohol dependence, but found limited effects of client heterogeneity. However, it is possible that the existing examinations of Project MATCH data have not fully characterized the nature of severity of alcohol dependence, as these analyses have typically examined dependence severity as an additive symptom count similar to the diagnostic strategy represented in the DSM-IV. We propose to examine dependence severity as a latent trait hypothesized to have a characteristic developmental progression, and that this conceptualization may result in a dimension that may represent an important source of variance in treatment response and outcome, both directly and as a moderator of other important typological variables. We seek to identify salient markers of dependence severity on this putative developmental continuum using techniques of Item Response Theory (IRT), and examine the implications of this approach to severity scaling in the Project MATCH data with respect to its effect on treatment and treatment matching outcomes. Specifically, this project seeks to (1) use IRT to establish scaled severity markers that may theoretically reflect milestones in the developmental course of alcoholism; (2) test the generalizability of the scaled continuum across gender and ethnic background; and (3) scale Project MATCH participants based upon their empirical fit with (traitedness) and their placement on (severity) this identified latent severity trait, and then use this information to re-examine the treatment effects and matching hypotheses central to that project.